The invention relates to a signal processing method, and more particularly, to elimination of pulse wave noise, which can be used in measuring an oxygen saturation through pulse photometry.
A pulse oximeter has hitherto been used for continuously and noninvasively measuring an oxygen saturation of arterial blood. In the case of the pulse oximeter, a probe is attached to a finger or ear lobe of an examinee, Red light and infrared light, which differ in wavelength from each other, are irradiated from the probe to a living body in the manner of time division. An oxygen saturation S is measured on the basis of a ratio "PHgr" between pulsation components of light absorbances obtained from transmitted or reflected light of the two different wavelengths. For instance, a wavelength of 660 nm is used for red light, and a wavelength of 940 nm is used for infrared light Two light-emitting diodes for emitting these wavelengths and one light-receiving photodiode are housed in the probe. Provided that a pulsation component of a light absorbance caused by pulsation of infrared light is taken as S1 and a pulsation component of a light absorbance caused by pulsation of red light is taken as S2, the ratio "PHgr" (hereinafter sometimes called simply an xe2x80x9cabsorbance ratio "PHgr"xe2x80x9d) is determined by the following equations.
"PHgr"=S2/S1xe2x80x83xe2x80x83(1)
xe2x80x83S=f("PHgr")xe2x80x83xe2x80x83(2)
In such a pulse oximeter, any movement of a patient during measurement will cause noise components to be mixed into a pulse wave to be detected by a probe, thereby hindering accurate measurement of an oxygen saturation S. To prevent such a case, conventionally, attempts have been made to eliminate the influence of such noise.
As described in, e.g., U.S. Pat. No. 5,632,272, pulsation components S1, S2 measured in determining an oxygen saturation are considered to include signal components s1, s2 and noise components n1, n2. There is determined a ratio of s1 to s2 such that a correlation between the signal component s1 and the noise component n1 becomes minimum.
However, in order to determine a ratio of S1 to s2 such that the correlation between the signal component s1 and the noise component n1 becomes minimum, computation must be performed while the value of the ratio is sequentially changed. As a result, the amount of computation becomes enormous, thus imposing considerable computation load on a measurement apparatus. In addition, computation processing takes much time, thus posing a problem on immediate computation.
It is therefore an object of the invention to reduce the amount of computation and immediately eliminating noise components from a measurement signal. In particular, the invention aims at eliminating noise from a pulse waveform on which noise has been superimposed for reasons of body movement, thereby reproducing an original pulse wave. It is also an object of the invention to provide an apparatus for accurately measuring an oxygen saturation through pulse photometry.
In order to achieve the above objects, according to the invention, there is provided a method of processing two continuous signals having an identical fundamental frequency, comprising the steps of:
providing a first signal and a second signal as the two continuous signals;
obtaining a first spectrum which is either one of a frequency spectrum or a frequency power spectrum of the first signal in a predetermined time period;
obtaining a second spectrum which is either one of a frequency spectrum or a frequency power spectrum of the second signal in the predetermined time period;
obtaining a first normalized value which corresponds a ratio of a difference between the first spectrum and the second spectrum and a sum of the first spectrum and the second spectrum at the fundamental frequency; and
obtaining a first ratio of an amplitude of a signal component of the first signal and an amplitude of a signal component of the second signal based on the first normalized value.
With such a method, the above first ratio can be determined even when the first signal and the second signal include noises.
Preferably, the signal processing method further comprises the steps of.
obtaining a second normalized value which corresponds a ratio of a difference between the first spectrum and the second spectrum and a sum of the first spectrum and the second spectrum at a noise fundamental frequency;
obtaining a second ratio of an amplitude of a noise component of the first signal and an amplitude of a noise component of the second signal based on the second normalized value; and
eliminating noises from at least one of the first signal and the second signal based on the first ratio and the second ratio.
With such a method, the noise components can be suitably eliminated to determine at least one of the amplitude of the signal component of the first signal and the amplitude of the signal component of the second signal.
Here, it is preferable that the signal processing method further comprises the step of displaying at least one of the first signal and the second signal in which the noises have been eliminated at the noise eliminating step.
In this case, there can be displayed pulse waves where noise components have been suitably eliminated.
Preferably, the first signal and the second signal are provided as data with respect to pulse waves measured by pulse photometry.
In this case, the above first and second ratios can be determined from the first and second signals measured by pulse photometry.
Here, it is preferable that the first signal is data with respect to a pulse wave of infrared light, and the second signal is data with respect to a pulse wave of red light, The signal processing method further comprises the step of obtaining an oxygen saturation based on the first ratio.
With such a method, an oxygen saturation can be measured with superior accuracy.